In the field of microfabrication represented by the manufacture of integrated circuit devices, lithographic technology enabling microfabrication with a line width of 0.20 μm or less has been demanded in order to increase the degree of integration.
A conventional lithographic process utilizes near ultraviolet rays such as an i-line radiation. However, it is very difficult to perform microfabrication with a line width of sub-quarter micron using near ultraviolet rays.
Therefore, in order to enable microfabrication with a line width of 0.20 μm or less, utilization of radiation with a shorter wavelength has been studied. Deep ultraviolet rays represented by a bright line spectrum of a mercury lamp and an excimer laser, X-rays, electron beams, and the like can be given as radiation with a shorter wavelength. Of these, a KrF excimer laser (wavelength: 248 nm) and an ArF excimer laser (wavelength: 193 nm) have attracted attention.
As a resist applicable to the excimer laser radiation, a number of resists utilizing a chemical amplification effect between a component having an acid-dissociable functional group and a component generating an acid (hereinafter referred to as “photoacid generator”) which generates an acid upon irradiation (hereinafter referred to as “exposure”) have been proposed. Such a resist is hereinafter called a chemically-amplified resist.
Japanese Patent Publication No. 2-27660 discloses a chemically-amplified resist containing a polymer having a t-butyl ester group of a carboxylic acid or t-butyl carbonate group of a phenol and a photoacid generator. The t-butyl ester group or t-butyl carbonate group in the polymer dissociates by the action of an acid generated upon exposure, whereby the polymer has an acidic group such as a carboxylic group or a phenolic hydroxyl group. As a result, exposed areas of the resist film become readily soluble in an alkaline developer.
Generally, conventional KrF chemically-amplified resists contain a phenol resin as a base resin. However, phenol resins are not adequate to be used in ArF photoresists due to their strong absorption of ArF laser rays by aromatic rings, such that a sufficient amount of ArF laser rays does not effectively reach a lower portion of the resist film so that the irradiation dose is increased in the upper portion of the resist film and decreased in the lower portion. As a result, the resist pattern after development is trapezoidol resulting in a resist pattern that is thinner in the upper portion and thicker in the lower portion. Sufficient resolution is generally not obtained. If the resist pattern after development is in the shape of a trapezoid, desired dimensional accuracy cannot be achieved in a succeeding step such as an etching step or ion implantation step. Moreover, if the shape of the upper part of the resist pattern is not rectangular, the rate of removal of the resist by dry etching is increased, whereby it is difficult to control etching conditions.
The shape of the resist pattern can be improved by increasing the radiation transmittance of the resist film. For example, a (meth)acrylate resin, e.g., a polymethylmethacrylate, is a highly desirable resin from the viewpoint of radiation transmittance, because the (meth)acrylate resin has high transparency to deep ultraviolet rays. Japanese Patent Application Laid-open No. 4-226461 discloses a chemically-amplified resist using a methacrylate resin, for example. However, this composition has insufficient dry etching resistance due to the absence of an aromatic ring, although the composition performs well in microfabrication performance. This makes it difficult to perform etching with high accuracy. Therefore, a composition having both transparency to radiation and dry etching resistance is not provided in this manner.
As a means to improve dry etching resistance of a chemically-amplified resist, without impairing transparency to radiation, a method of introducing an aliphatic ring into the resin component in the resist polymer instead of an aromatic ring has been studied. For example, Japanese Patent Application Laid-open No. 7-234511 discloses a chemically-amplified resist using a (meth)acrylate resin having an aliphatic ring.
To further improve ArF photoresist performance, one or more recurring unit(s) has been introduced to the above described resins. For example, Japanese Patent 3042618 discloses a chemically-amplified resist using a resin by incorporating a recurring unit having a lactone skeleton. Japanese Patent Application Laid-open No. 2002-296783A discloses a chemically-amplified resist using a resin by incorporating recurring unit rather than described above.
However, the (meth)acrylate based photoresist resin discussed so far are prepared by conventional free radical polymerization processes. As the monomers therein are quite different in both molecular size and polarity for their chemical structures, copolymerization of them by conventional free radical polymerization generates several disadvantages in the molecular characterizations of the resin: (1) wide polydispersion (2) monomer drift between polymer chains (3) difficulty in control of polymerization reproductivity.
Therefore, a need exists for polymeric resins which overcome one or more of the above-identified drawbacks of currently available photoresist polymeric resins.